Field of the Invention
The present invention relates to a device for measuring the flow rate of a fluid circulating in pipes.
The so-called "voludeprimometer" method is the most commonly used for measuring the instantaneous flow rate of a fluid, liquid or gas. For this purpose, a flow measuring device, called hereinafter a flowmeter, is inserted into the pipe where the fluid is circulating. This type of flowmeter includes a member which creates a differential pressure between the upstream flow path of the pipe and the downstream flow path. This member is usually, for example, of the diaphragm or nozzle type having a metering hole traversed by the fluid and thus creating the differential pressure. Pressure inlets constituted by pipes issuing on either side of the pressure differential member transmit the pressures in the upstream circuit and downstream circuit to a suitable pick-off or sensor, e.g. of the mechanical or electromechanical type.
Pressure differential members are static elements of simple construction, which are therefore generally inexpensive and easy to modify. They are used under very high pressures. However, the equation linking the differential pressure with the flow rate is of the form Q=k.sqroot..DELTA.P in which Q is the volume flow rate, k a parameter dependent on the measuring conditions and the dimensional characteristics of the pressure differential member and .DELTA.P the differential pressure in the upstream circuit and the downstream circuit. Thus, the flowmeter must be equipped with a mechanism able to linearize this function. If the flowmeter is used as a simple measuring apparatus, these mechanisms can be reduced to their most simple expression by giving the measuring apparatus a quadratic reading scale. However, the accuracy at the bottom of the range is low. However, if the flowmeter is used as a control device, for example incorporated into a control chain, more elaborate linearization devices must be provided. They can be mechanical devices, e.g. cams shaped according to a law of the second degree or electrical members, e.g. potentiometers or similar components, whose resistance varies according to the same law.
It is in fact desirable to be able to obtain a measuring signal which is directly proportional to the flow rate. On the one hand, a flow indicator can be graduated in a regular manner and on the other this obviates the introduction of a non-linearity which can have an unfavourable influence on the dynamic behaviour of a control loop. Finally, these mechanical or electromechanical conversion members incorporate moving parts which are relatively unreliable, cumbersome and have a slow response.
One of the interesting applications of such a flowmeter is the measurement of the quantity of air admitted into the cylinders of an internal combustion engine. The knowledge of this parameter is necessary for the construction of certain types of regulation systems used for fuel injection purposes. The tendency at present is to use systems of the digital type using, for example, a central processing unit for the data corresponding to the different parameters measured and constituted by a microprocessor. It is therefore of interest to use sensors supplying digital signals. In addition, the linearization of signals involving a quadratic law is facilitated. This linearization can either be carried out by a cabled device generally using pulse counters, or by the actual processor by means of a suitable program.